As the information technology is developing, dramatic innovation on information technology, particularly on magnetic recording technology, is desired more and more. In a magnetic disk that is to be installed in a hard disk drive (HDD) being a type of magnetic disk device used as a computer storage, the information recording density is being increased rapidly, unlike in other types of magnetic recording media, such as magnetic tapes and flexible disks. Accordingly, the capacity for storing information in a personal computer is dramatically increasing on the strength of the increase in information recording density of the magnetic disk.
The magnetic disk includes a magnetic layer and other layers on a substrate, such as an aluminum-based alloy substrate or a glass substrate. In a hard disk drive, the magnetic disk is rapidly spun under a flying magnetic head, and the magnetic head records information signals as magnetized patterns on the magnetic layer, or reproduces the recorded information signals.
As the demand (for mobile use) that the hard disk drive is used in portable apparatuses (for example, notebook personal computer devices) increases, attention is directed to glass substrates having high strength, high stiffness, and high impact resistance for the magnetic disk substrate. In addition, the glass substrate can have a smooth surface. Accordingly the glass substrate facilitates the reduction of the flying height of the magnetic head that records and reproduces information while being floating over the magnetic disk. Thus, a magnetic disk having a high information recording density can be achieved.
However, the glass substrate is made of a brittle material. Accordingly, a variety of approaches have been proposed to strengthen the glass substrate. For example, Japanese Patent (JP-B) No. 2657967 (Reference Document 1) has described chemical strengthening treatment in which the glass substrate is immersed in a mixed solution of KNO3 and NaNO3 for a predetermined time to substitute K+ ions for Li+ ions at the surfaces of the glass substrate and thus to form compressive stress layers at both main surfaces and a tensile stress layer between the compressive stress layers. Reference Document 1 has taught that the maximum tensile stress of the tensile stress layer is preferably 4 kg/mm2 or less.
Japanese Patent (JP-B) No. 3254157 (Reference Document 2) has disclosed that when the glass substrate chemically strengthened by the same method as in Reference Document 1 has a thickness of 0.5 to 1.0 mm, the compressive stress layers preferably have a thickness of 80 to 100 μm and a compressive stress of 2 to 15 kg/mm2 with the tensile stress layer having a tensile stress of 1.5 kg/mm2 or less.
The information recording density of the magnetic disk has been increased as high as over 40 gigabits per square inch in recent years. Further, a super high recording density of over 100 gigabits per square inch is about to be realized. The recent magnetic disk exhibiting such a high information recording density can store a practically sufficient amount of information even if it has a much smaller area than known magnetic disks.
The magnetic disk has a much higher information recording and reproduction speed (response speed) than other information recording media, and can accordingly record and reproduce information anytime.
These features of the magnetic disk arouse a demand for such a small hard disk drive as can be installed in portable apparatuses much smaller than personal computers and requiring high response speed, such as cellular phones, digital cameras, portable information apparatuses (for example, PDA's (personal digital assistants)), and car navigation systems. Specifically, highly mobile apparatuses require a small hard disk drive containing a magnetic disk using a substrate having, for example, a diameter of 50 mm or less, or of 30 mm or less, and a thickness of less than 0.5 mm, or of 0.4 mm or less, and such highly mobile apparatuses include portable information apparatuses such as cellular phones, digital cameras, portable MP3 players, and PDA's, and vehicle-mounted apparatuses such as car navigation systems.
The small hard disk drive used in these portable or mobile apparatuses is always exposed to the risk of impact from falling or vibration. Accordingly, the hard disk drive for these applications, including the magnetic disk, requires that each internal member has higher impact resistance so as to enhance the reliability.
The magnetic disk using a glass substrate is useful for hard disk drives used in the portable apparatuses. This is because the hard substrate made of glass has higher stiffness than metals, which are rather soft, and because the strength of the glass substrate can be increased to a desired level by chemical strengthening treatment or the like as described above.